Erration storage system with collimated electron beam for minimal spherical ab

ABSTRACT

A storage system for the mass recording and readout of digital data with ultra high resolution. An electron beam structure is provided for forming a beam of extremely small focused spot diameter, on the order of 0.1 microns, and high current density capability, on the order of 1,000 amperes per sq. cm., which records data by scanning over defined areas of the storage medium surface and micromachining elemental portions of said medium as a function of beam modulation. Readout may be subsequently accomplished by similarly scanning the beam at reduced power density and detecting electrons that have been transmitted by or reflected from the storage medium.

limited States Patent 1 Wolfe et a1.

[4 1 Sept. 18, 1973 STORAGE SYSTEM WITH COLLIMATED ELECTRON BEAM FORMINIMAL SPHERICAL ABERRATION inventors: John E. Wolfe, Camillus; GeorgeE. Ledge, Liverpool; Homer H.

Glascock, Scotia, all of NY.

As si gneei "GiiEi'E'iEEic Cdrnpany,

Syracuse, NY.

Filed? Jm ""1,'r97r M "W App]. No.: 158,763

Related US. Application Data Division of Ser. No. 847,972, Aug. 6, 1969.

References Cited UNITED STATES PATENTS 5/1970 Dove 340/173 CR PrimaryExaminer-Stanley M. Urynowicz, Jr. Assistant Examiner-Stuart N. HeckerAttorney-Richard W. Lang et al.

[57] ABSTRACT A storage system for the mass recording and readout ofdigital data with ultra high resolution. An electron beam structure isprovided for forming a beam of extremely small focused spot diameter, onthe order of 0.1 microns, and high current density capability, on theorder of 1,000 amperes per sq. cm., which records data by scanning overdefined areas of the storage medium surface and micromachining elementalportions of said medium as a function of beam modulation. Readout may besubsequently accomplished by similarly scanning the beam at reducedpower density and detecting electrons that have been transmitted by orreflected from the storage medium.

16 Claims, 11] Drawing Figures PATENTED SEP] 8 I973 SHEET 3 BF 3 FIGJO YSTORAGE SYSTEM WITH COLLIMATED ELECTRON BEAM FOR MINIMAL SPHERICALABERRA'IION This is a division of application Ser. No. 847,072, filedAug. 6, 1969.

BACKGROUND OF THE INVENTION ing which also have extremely high currentdensity characteristics for a permanent data storage.

pacity of this magnitude is considered desirable in order to satisfypresent day archival storage requirements and workers in the fiedlcontinue to be Pngaged in finding the most effective means for itsaccomplishement within a confined space. The principal limitation ofstorage systems developed to date is the resolution of the storagedata.Lacking veryhigh resolution, these systems require extensive storagespace for storing large quantities of data and access times to said dataare necessarily'slow. In order to index'elemental data bits for eitherstorage or retrieval purposes, a relatively complex and slow mechanicalaccessing operation is normally required. In one system known to the arta laser beam is employed for recording and reading out data on aphotosensitive storage medium which is in the form of a long continuoustape. Information'is written by scanning the beam diagonally across thetape as the tape moves longitudinally, providing parallel scan linesthroughout the tapes length. Because of inherent limitations in thewavelength of laser energy, on the order of 5,000 to 6,000 angstroms,data can be recorded. with resolutions no greater than a few microns.Thus, for a bit memory, 2,400 feet of 8 mm tape must be provided. It maybe appreciated that lengthy access times are required for this system.Another-existing approach is to employ anelectron SUMMARY OF THEINVENTION It is accordingly a principal object of this invention toprovide a novel ultra high density storage'system which permanently anddirectly records data at appreciably higher resolutions than presentlyobtainable, making possible the rapid storage of huge quantites of datawithin a confined space.

It is a further object of the invention to provide a novel storagesystem as above described wherein recording and readout operations areaccomplished at high speed, and do not require a separate developingstep.

A further object of the invention is to provide a novel storage systemas described wherein exceedingly large quantites of data, on the orderof 10 bits and greater, can be stored on a single, relatively small,storage surface.

' It is still another object of the invention to provide a novel ultrahigh density storage system as above described inwhich data is recordedby micromachining elemental portions of the storage medium by means of ascanned electron beam,the stored data being read out also by a scannedelectron beam.

A still further object of the invention is to provide a novel electronbeam forming structure for generating a beam of extremely small spotdiameter, on the order of 0.1 microns, and high current density, ontheorder of 1,000 amperes per sq. cm.

. A yet furtherobject of. the invention is to provide a novel electronbeam forming structure for generating a beam with theabovenotedcharacteristics by means of a relatively low acceleratingvoltage, on the order of 5 to 10 KV.

It is another object of theinvention to provide a novel electronemission system having a field aided thermionic cathode that emitselectrons with'an exceedingly high current densityand is extremelystable in its operation.

beam for writing on photographic film.'This has been accomplishedemploying a beam having a three micron resolution and recording on 35mm-chips. For a 10" bit storage capability there are'required 200,000individual chips. Themechanical accessing requirements for this systemare extremely complex. Further, it isnecessary to develop thephotographic film before the information can be read out or checked, anddata cannot be subsequently entered. 1 1

In the field of electron microscopyit has been suggested to employsscanned electron beam ofextremely small spot diameter, such asp'resentlyutilized in scanning electron microscopes, to record digitalinformation by means of selective etching or a-comparable technique.The-art has been developed to where there presently existbeam formingapparatus which generate beam spot diameters on the order of severalhundred angstroms and less, corresponding to a resolution orders ofmagnitude higher than in the abovenoted systems. However, these are allrelatively low power density beams, not capable of providing directlypermanent data storage using presently available recording materials.Accordingly, there does not exist at the presout time apparatus forgenerating electron beams of minute dimensions for an ultra highresolution record- Still another object of the invention is to provide anovel field aided thermionic cathode employing a tungsten needle coatedwithan atomic layer of zirconium .which exhibits an extremely longlifetime, on the order of 1,000 hours. and greater.

which the effective spherical aberration of the focus lenses is made lowfor relatively large focal lengths. In accordance with these and otherobjects of the invention there is provided an ultra high density storagesystem which employs a scanned electron beam of extremely small spotsize and high current density to record data on a storage medium bymicrom achining elemental portions of said medium. Readout of the storeddata is accomplished by means of the scanned electron beam, modulatedelectrons from the target storage r'nedium being collected by a detectordevice. The readout electron beam is at reduced current densities whichwill not destroy the'stored data bits. For high capacity stor-.

age, data is stored as numerous discrete data blocks over each of whichthe electron beams can be magnetically or electrically scanned.Mechanical drive means are provided for indexing the data blocks withrespect to said beam for both write and readout operations.

With respect to one specific aspect of the invention, the electron beamis produced by a novel electron emission system through a process offield aided therm ionic emission. The electron emission system iscomposed of a cathode including a filamentary hairpin with a weldedsingle crystal oriented tungsten needle which is of extremely smalldimension at the emissive cathode tip. Sintered zirconium is applied ina ball at the base of the needle, and upon heating of the hairpin andneedle migrates as a solid up the needle to the tip. The zirconiumcoating acts to reduce the work function of the 100 face on the tip ofthe tungsten crystal to a value appreciably lower than occurring onother faces. The emission system further includes an apertured anode andgrid electrode structure for generating a spherical electric fieldconfiguration about the emissive cathode tip which exhibits a very highfield gradient at said tip for causing a high power density electronemission.

With respect to a second specific aspect of the invention, an electronoptical system is provided which includes first and second focus lensesto provide a single stage imaging of the electrons emitted-from thecathode tip onto the target. The cathode tip, which is at about theobject plane, is positioned at the focal point of the first lens and thetarget, which is at the image plane, is positioned at the focal point ofthe second lens. The focused beam impinges on the target at sufficientlyhigh power densities to varporize away portions of the target material.Modulation of the beam is accomplished by a modulation coil which shiftsthe beam axis with respect to a limiting aperture provided in thevicinity of the beam source. A set of deflection coils are providedforward of the final focusing lens for deflecting the beam in both X andY directions in the plane of the storage medium.

BRIEF DESCRIPTION OF THE DRAWING- The specification concludes withclaims particularly pointing out anddistinctly claiming the subjectmatter which is regarded as the invention. It is believed, however, thatboth as to its organization and method of operation, togetherwith'further objects and advantages thereof, the invention may be bestunderstood from the description of of the preferred embodiments, takenin connection with the accompanying drawings in which:

FIG. I is a schematic diagram in apartially broken away perspective viewof an electron beam ultra high density storage system in accordance withone embodiment of the invention employing a limited area storage medium;

FIG. 2 is an enlarged side view of the storage medium and electrondetector employed in the system of FIG. 1;

FIG. 3 is a partial plan view of the storage medium employed in thesystem of FIG. 1, illustrating the written data format;

FIG. 4 is a series of graphs illustrating the formation of the inputmodulation signal;

FIG. 5 is a side view of a modified reflective readout structure;

FIG. 6 is a detailed cross sectional view of the total electron beamstructure of FIG. 1;

FIG. 7 is an enlarged cross sectional view of the electron emissionstructure of FIG. 6;

FIG. 8 is a further enlarged cross sectional view of the cathodestructure;

FIG. 9 are several curves illustrating field aided thermionic emission;

FIG. 10 is a schematic diagram in partially broken away perspective viewof a further embodiment of the invention employing a large area storagemedium and mechanical drive means for indexing said storage medium; and

FIG. 11 is a partial plan view of the storage medium employed in thesystem of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. I, there isschematically illustrated in perspective view an electron beam storagesystem for permanently storing data at ultra high densities, exceeding10 data bits per sq. cm. The data is written by a scanned electron beamof extremely small beam spot diameter, on the order of 0.1 micron, andextremely high current density. A magnetic deflection was employed inthe illustrated system, although an electrostatic deflection might alsobe used. The writing beam,

at the focused spot, exhibits a current density of 10 amperes per sq.cm. and greater, and therefore a power density of 10 watts per sq. cm.and greater withinmoderate anode voltages. The writing beam is modulatedas a function of the input data so as to selectively micromachine byvaporization elemental portions of the medium 1 as the beam is scannedover its surface. Scan rates as high as 10 data bits per second andhigher are employed. A non-destructive readout of the stored data isaccomplished by a scanned readout electron beam operated at reducedpower levels, about one tenth that of the writing beam.

The electron beam structure for both record and readout includes anelectron emission system 3 and an electron optical system 4 associatedwith an evacuated chamber 5 within which the beam is enclosed. Theemission system 3 produces the electron beam and includes among itsprincipal components a cathode structure 7, grid electrode 8 and anodeelectrode 9 The electron opticalsystem 4 controls, focuses and deflectsthe beam and principally includes focusing coils 10A and 10B, deflectioncoils l1 and modulation coils 12. The emission system 3 and electronoptical system 4 include features of novelty that make possible the extremely high resolution, high current density properties of the focusedelectron beam, and will be described in greater detail when consideringFIGS. 6 and 7. An input means 13, 'shown in block form, supplies .inputsignals for modulating thebeam. Input means 13 may comprise aconventional source of digital data, e.g., the peripheral equipment of adigital computer. Although, in the disclosed embodiments of theinvention the modulation signal contains digital data,the invention neednot be so limited andmay be useful with other data forms such as analogand alphanumeric data.

A medium vacuum, onthe order of IO mm. Hg, is provided within the.chamber 5 by a vacuum pump 14, schematically illustrated in block form.A vacuum of this magnitude" represents a compromise that it isrelatively easy to achieve and maintain, while being compatible with thehigh order of performance required of the present system. Also housedwithin the chamber 5 is the readout structure in the form of an electrondetector upon which the storage medium 1 is deposited. The storagemedium 1 and detector 15 are mounted on a base structure 16. An outputmeans 17 is connected to the detector 15 for receiving the readout data.A master control logic network 18 is coupled to input means 13 andoutput means 17. The network 18 includes, numerous logic circuits ofstandard design for providing the sundry logic functions which controlthe writing and readout operations, as well as a clock frequencygenerator for supplying a master timing of said operations.

The electron detector 15 may be a conventional component. In theembodiment being considered it is a single crystal silicon p-i-njunction device which in response to penetrating electrons of thereadout beam generates electron-hole pairs. As shown in the side view ofFIG. 2, the silicon detector includes a p region 20, an intrinsic region21 and an n region 22, with contacts 23 and 24 made to the p and nregions, respectively. The storage medium 1 is deposited as a thin filmover the p region 20. A d.c. voltage source 25 is connected through alead resistor 26 to contacts 23 and 24 for reverse biasing the device.Output terminals 27 are coupled through a capacitor 28 to contacts 23and 24 for sensing readout current flow through the device 15.

During the write operation the electron beam is operated at high currentand power densities. As it scans across the surface of the storagemedium 1, the beam selectively micromachines elemental portions thereof,corresponding to the data bits, as a function of beam modulation.Aheating and rapid vaporization of said elemental portions of thematerial occur in response to penetration by the beam s high velocityelectrons. With respect to the vaporization process, the high densityelectrons of the focused beam spot penetrate the storage-material withrelatively high kinetic energy. As the electrons are rapidly deceleratedby the bulk of the material, heat is given off which in a localized areaelevates the temperature to a point well above the threshold temperatureof the vapor pressure versus temperature curve of the material, thethreshold temperature being that temperature at which the vapor pressurecommences to rise steeply as a function of tempera ture. At thiselevated temperature, the vapor pressure of said localized area israised orders of magnitude above the ambient pressure and rapidvaporization of the material results.

it is preferred to modulate the beam by applying the input signal to themodulation coils 12 for recurrently shifting the beam from its centralaxis so as to be partially intercepted during its travel, therebymaintaining a constant current emission while modulating the currentdensity at the focused spot. The beam is rapidly scanned by thedeflection coils 11 along parallel data tracks successively formed onthe surface of the storage medium.

In the embodiment of FIG. 1, the storage medium 1 has a storage areaabout 36 mils square with about 4,500 data lines and about 4,500resolvable elements per data line. Thus, there is provided a storagecapacity in excess of 2 X 10 bits. With reference to FIG. 3, a smallarea of the storage medium 1 in a greatly magnified plan view isillustrated, showing four data strips 31, 32, 33 and 34, the edges ofwhich-comprise the data lines. Tracks 35, formed between the datastrips, are

employed to servo the readout beam as wil' be further- 6 explained.Information is written along both edges of each data strip at resolvableelements on the data lines, such as shown with respect to resolvableelements m and n. In accordance with standard practice, the data lineshave a string of known data bits written at the be-- ginning of eachline which are used as a reference during readout. In the present formatthe data strips are on centers spaced apart by 0.4 microns, and eachresolvable element is 0.2 microns in length. Digital information ofbinary l s and 0 s is written as a 180 phase modulation of a square waveat one half the reference clock frequency supplied by the logic network18, so as to correspondingly micromachine the data strips during eitherthe first or second half of travel of the beam through each resolvableelement. The writing beam may vaporize essentially the entire thicknessof the storage medium or only a fraction of this thickness.

Referring to the diagram of FIG. 4, the binary bits may be fed in as aseries of l s and 0 s at two corresponding d.c. levels, as shown by theGraph A. It will be assumed that the bits are supplied at a 10 MHz rate.The clock signal, shown in Graph B, is at twice the data rate or 20 MHz.Graph C illustrates the phase modulated square wave corresponding to thedata bits'in Graph A that is employed as the'input signal formodulating' the beam. With clock signal at a 20 MHz rate, a single databit is written in 0.1 microseconds.

For the operation under consideration, the storage medium 1 must be astable material capable of being selectively and rapidly vaporized atthe requisite resolution of the system by thewriting electron beam, andyet be totally unaffected by the lower energy readout beam. Principalproperties of the medium '1 include a relativelyhigh vapor pressure atthe writing temperature, and a vapor pressure that is a steep functionof temperature above threshold; a high density; and a low thermalconductivity. A high vapor pressure of the writing temperature causesthe material to rapidly vaporize in.response to heating by the writingbeam. The steep vapor pressure versus temperature function permits areduced power readout operation that can produce no vaporization of thematerial. High density and low thermal conductivity properties permit alocalized heating of thematerial necessary for high resolution writing.The assignment of specific values for the above noted properties isdependent upon the writing and readout beam parameters as well as systemrequirements for resolution, speed, etc., and the interrelationship ofthese factors. For example, the requirements for high vapor pressure,high density and low thermal conductivity are inversely related to thewriting beam current density.

In accordance withexisting performance specifications fora given system,the storage medium may be selected from various classes of materialsincluding semimetallic, semiconductor and dielectric'materials. [n theembodiment under consideration there is employed an alloy of seleniumwith 10-20 percent arsenic for retaining an amorphous form of theselenium. This material has a specific gravity of about 4.3, a thermalconductivity of about 10 calories per sec. per cmC, anda vapor pressureof about 10 mm. Hg. at a writing temperature of 700C, which pressurereduces to about 10" mm. Hg. at a readout temperature of C. The materialis deposited on one surface of the electron detector 8 as a thin film,having a thickness of approximately 1,500 to 3,000 A.

Considering the readout operation, the electron beam is operated atreduced power density. This may be accomplished by several differentmeans, but it is preferable to reduce the current density of the focusedspot through partial interception of the beam by biasing the beam offits central axis. The reduced power density causes a correspondinglyreduced heating of the storage medium, and a very greatly reduced vaporpressure, as previously noted. In the embodiment of FIG. 1, as thereadout beam is scanned the electrons from the beam are transmittedthrough the etched portions of the data elements and penetrate theelectron detector 15. Electron-hole pairs are created by the penetratingelectrons which generate a corresponding readout signal from outputmeans 17. The readout signal contains the readout data in the form ofphase information similar to the input signal. To maintain accuracy ofthe readout signal, this signal is synchronized with the clock frequencyduring the readout of each data bit.

In order that the readout beam precisely follow each data line, a servosystem is provided for sensing and correcting any tendency for beamoffset. One of several conventional servo techniques may be employed. Inthe storage system of FIG. 1, an edge servo system is used wherein asthe beam is scanned along a data line, displacement from the edge of thedata track is sensed and a correction signal generated. A servo unit,including a low pass filter and an error sensor to which the readoutsignal is coupled, may be embodied within the output means 17. As thebeam travel may tend to deviate from a scanned data line toward or awayfrom the adjoining servo track, a low frequency component is introducedinto the readout signal the magnitude of which is a function of the beamdisplacement. In response to said low frequency component, the servounit generates a correction signal which is coupled to the verticaldeflection coils for compensating the beam's travel. I

In FIG. is illustrated an alternate embodiment of the readout structureof the reflective type. In this embodiment, a storage medium 41 isdeposited upon a supporting substrate 42, e.g., of glass. An electrondetector 43 is positioned above the storage surface and offset from theimpinging electron beam. The electron detector 43, which may be one ofseveral conventional types including the p-i-n junction deviceillustrated in FIG. 2, receives readout electrons which are reflectedfrom the storage medium surface. The back scattered electrons can bereflected primary or secondary electrons, or both. An acceleratingpotential, not shown, is applied in known fashion to the detector forsensing secondary electrons. The readout operation isotherwise similarto that previously described, the detector responding to the reflectedelectrons for generating electron-hole pairs within its volume. which inturn provides a corresponding readout signal. Other forms of electrondetectors such as channel multipliers and photon devices may also beemployed.

A cross sectional view of the total electron beam structure includingthe electron emission system 3 and the electron optical system 4 isshown in FIG. 6, taken along the plane 6-6 in FIG. 1. An enlarged crosssectional view of the electron emission system per se is illustrated inFIG. 7, and a further enlarged view of the cathode structure is shown inFIG. 8. The electron beam structure of FIG. 6 and 7 forms an electronbeam having a theoretical current density j at the focused spot on thetarget that may be defined by Langmuirs equation as follows:

where j, is the emission current density at the cathode emissivesurface;

e is the electron charge;

V is the voltage at the target;

K is Boltzmans constant;

T is the absolute temperature; and

at is the half angle at the focused spot.

With reference to the above equation, it may be appreciated that-therequirements of the storage system impose a number of significantconstraints on the electron beam structure in providing extremely hightarget current densities. Thus, a prime consideration for obtaining ahigh target current density j is to maximize the cathode emissioncurrent density j The current den- .sity j is also proportional to thetarget voltage V. However, the voltage V also determines the velocity atwhich the electrons strike the target and an excessively high voltagewill result in expanding the elemental heated portions of the storagemedium and degrading resolution. Thus, the value of V must be determinedwith these conflicting considerations in mind. From the above equationit is also seen that the current density j is inversely proportional tothe temperature T, which places a limitation on heating of the cathode.

Referring to FIG. 7, the cathode structure 7 includes a hairpin filament50 having a cathode needle 51 joined at the vertex of said hairpin. Apotentiometer, including a dc. source 52 in shunt with a resistor 53, iscoupled to the terminals of the filament 50 for heating said filament. Anegative high voltage source -V, is coupled to a tap on the resistor 53.A shield 54 surrounds the cathode, grid and a part of the anodestructure. The grid electrode 8 is in the form of a disk having anaperture 55 through which the cathode needle extends. A negative voltagesource V, is coupled to the grid 8, where V is slightly more negativethan -V,. The anode electrode 9 is of the re-entrant type, the reentrantportion being provided with a central aperture 56 positioned immediatelyforward of the cathode tip. The anode electrode 9 is at groundpotential, as is all structure forward of anode. At the opposite orforward end of the anode electrode is a limiting apertured electrode 57in the shape of a disk having a central limiting aperture 58. Acylindrical sleeve 59 encloses the described emission structure. Severalpassages in the grid and anode structure, such as at 67, 68 and 69,facilitate evacuation of the electron emission region. The anodeelectrode, grid electrode and cathode needle structure together with thepotentials applied thereto produce a hemispherical electric fieldconfiguration around the cathode tip, with the tip at the radial centerof the hemisphere. The hemispherically configured electric field incombination with the extremely small dimensions of the cathode tipproduce a very high electric field gradient in the vicinity of said tip.The hemispherical field also limits aberrations in the focused beam.

In one operable structure, in accordance with the invention, the cathodeneedle 51 is about 30 mils in length and extends forward of the gridelectrode 8 by about 10 mils. This is the dimension g in FIG. 8. Thegrid electrode shields the hairpin and assists in limiting emission tothe tip of the cathodeneedle, as well as in shaping the hemisphericalelectric field. The emissive surface at the cathode needle tip has aradius of about 1 micron. The grid electrode aperture 55 is about 10mils in diameter. The anode electrode 9 is about 30 mils forward of thegrid electrode 8, shown as the dimension h in FIG. 8. The anodeelectrode is about 1.125 in. wide at the forward end and has a totallength in the axial direction of about 1.1 in. The anode aperture 56 isabout 10 mils in diameter and the limiting aperture 58 is about 20 milsin diameter. For the indicated length, dimension and forward extensionof the cathode needle 51, the grid to anode spacing and the dimensionsof the grid, anode and limiting apertures, the voltage V was at 5.0 KVand the voltage V at 5.3 KV. An electric field gradient of 10 V/cm, wasthereby provided at the cathode tip. For a voltage V, of 10.0 KV and Vof 10.3 KV, the grid to anode spacing is modified, computed to be about40 mils, for retaining the 10 V/cm. electric field gradient at thecathode tip. The filament was heated to a temperature of approximately1,800I(. This temperature keeps the cathode tip clean of contaminatingadsorbed atoms in the medium vacuum that is used. The extremely highelectric field gradient in combination with heating of the filament 50produces a high density field aided thermionic emission from the cathodetip. It is noted that the high field gradient of 10 V/cm. is obtainedwith a moderate anode voltage of less than KV to about KV. These valuesof.voltage,.particularly at the lower end, are found notto provide anexcessively great velocity of electrons striking the present targetwhich might cause diffuse heating of the target-such as to degraderesolution. In addition, for extremelythin targets, overly high velocityelectrons may penetrate completely through and not generate sufficientheat in the target material.

In accordance with the operable embodiment under consideratiomthehairpin filament 50 is composed of rhenium selected for its refractoryand ductile characteristics. The filament has a diameter of about 10mils reduced to 7 mils at the vertex, as indicated in the enlargeddrawing of FIG. 8. The cathode needle Sl is an oriented single crystaltungsten having the 100 crystal face at the needle tip, which is thepreferential face for lowering the work function. The 100 crystal faceis orthogonally related to the needle longitudinal axis within a onedegree limit, preferably. The needle 51 is welded to the filament 50. Aslurry of zirconium hydride is applied as a bead to the base of theneedle 51 around the weld point. Upon heating of the filament, thezirconium hydride becomes sintere'd to form zirconium. The zirzoniummigrates over the surface of the needle and covers the tip, providingcontinuous replenishment for the effects'of evaporation and ionbombardment. An atomic layer of zirconium is thereby coated over thesurface of the needle 51 which, together with oxygen atoms from theresidual gas in the vacuum, act to reduce the work function at theemissive tip from 4.5 ev for pure tungsten to 2.8 ev. The reduced crosssectional dimension of the filament 50 at the. vertex raises thetemperature of this region relative to the remaining length of thefilament and assures migration of the'zirconium along the needle 51 inthe direction of the tip. The amount of zirconium material that need bedispensed is very little. A filament temperature of 1,800K in a mediumvacuum of about .10" mm. Hg keeps the cathode tip clean of adsorbedatoms. The described structure results in cathode lifetime that isextremely long, e.g., on the order of 1,000 hours and greater. It isnoted that the optimum filament temperature is a function of pressureand for medium vacuum may exist in a range, typically, of l,750 K to1,850K.

In FIG. 9 there are illustrated several field aided thermionic emissivecurves for both zirconium coated tungsten cathodes and plain tungstencathodes at dif- I ferent filament temperatures and for a fixed vacuum.The curves are plotted as emission current density in amperes per sq.cm. vs. electric field gradient in volts per cm. Curve A represents apure tungsten cathode heated to a temperature of 2,000K. The curve isseen to cross the 10 V/cm. field gradient line at a current density ofabout 10 amperes per sq. cm. Curve B represents a zirconium coatedtungsten cathode heated to a temperature of 1,500I(, which is seen tointersect the 10" V/cm. line at a current density of about 200 amperesper sq. cm. It is noted that although the filament temperature is lowerthan for curve A, the lowered work function of the zirconium coatedtungsten element appreciably increases the current emission. Curve Crepresents a pure tungsten cathode heated to a temperature of 2,600 K,which crosses the 10 V/cm. line at a current density of about 500amperes per sq. cm. It is seen that the elevated filament temperatureraises the current emission of the pure tungsten cathode above that ofcurves A and B. Curve D represents a pure tungsten cathode heated to atemperature of 3,000K,.which provides an emission current density inexcess of 1,000 amperes per sq. cm. at the 10" V/cm line. Although highemission densities are achieved, the temperature of curves C and D arefound to be excessively highso as to drastically limit the lifetime ofthe cathode. Curve E represents a zirconium coated tungsten cathodeheated to atemperature of 1,800K, which is the type employed in thedescribed embodiment. It is seen that this curve attains an emissioncurrent density only slightly less than that of curve D but at a verymuch lower temperature. Thus, at 1,800K it is foundv that a highemission density and high target current density is attained, and astable operation with long lifetime provided.

.Referring again to FIG. 6, a pair of modulation coils 12 of standarddesign are mounted on opposing surfaces of a cylindrical sleeve 59 ofthe vacuum chamber 5, the sleeve being shown also in FIG. 7.Themodulation coils are employed to direct the beam along a single axisin the X-Y plane, which is a plane transverse to the central axis Z ofthe beam. Forward of the anode electrode 9 there is mounted a firstmagnetic focus lens in the form of focus coil 10A which'is wound aboutthe circumference of the chamber 5 and produces a magnetic fieldpredominantly along the central axis of the beam. The coil 10A is per seof conventional type with its conductors enclosed by a magnetic ringcoil form. A gap 60 in the inner wall of the magnetic form locates thereference plane of thefocus lens, which is at the middle of the gap atplane 61. The reference plane is used for spatially relating the focuscoils one to the other and to the'object and image planes. The referenceplane is used for this purpose rather thanthe concept of principal planebecause for these lenses the.

principal planes are not readily located. Forward of the first focuscoil 10A is a second focus coil 108 similar to the first, having app 62in the coil form that places the reference plane of the second focuslens at plane 63. An astigmator coil 64, of standard design, is'woundabout the chamber for generating a proper axial magnetic field in theregion of the the limiting aperture 58. The astigmator coil is employedto compensate any astigmatism that may be produced by the focus coils Aand 1013. In addition, two pairs of centering coils 65, mounted onopposing surfaces of the vacuum chamber wall 66 in the vicinity of thelens plane 61 are provided for directing the beam along two orthogonallydisposed axes in the X-Y plane. The centering coils adjust the beam topass through the center of the second focus lens. Two pair of deflectioncoils 11 mounted on opposing surfaces of the wall 66 forward of thesecond focus coil 10B deflect the beam in two orthogonal directions inthe X-Y plane.

Electron optics principles of the structure shown in FIGS. 6 and 7 willnow be discussed. Electrons emitted from the emissive surface at thecathode emission under the hemispherical electric field configurationwill generally be directed along diverging paths corresponding to radiiof the hemispherical electric field, said paths appearing to emanatefrom a point slightly behind the cathode emissive surface which may beconsidered as a virtual image of the cathode. Only a fraction of theemitted electrons are passed by the anode aperture 56, the passedelectrons being within about a 10 solid angle of the beam central axis.0f the electrons transmitted through the anode aperture 56 only a smallfraction, within a solid angle of about 1, are passed by the limitingaperture 58. The first focus coil 10A transposes the diverging beam intoa collimated beam. The second focus coil 10B transposes the parallelbeam into a converging beam which is focused on the surface of thestorage medium.

Spherical aberration of a focus lens, C, is the most serious form oferror existing in electron optical systems, in general, with respect toproviding a sharply focused image. C, is primarily a function of lenspower, structural dimensions of the lens and accelerating voltage.Significantly, C, is inversely related to the lens power, or otherwiseconsidered, a direct function of the lens focal length. The presentconfiguration of the electron optical system very appreciably reducesspherical aberration of the system by minimizing the effective sphericalaberration C', of each lens. C', is defined as follows:

Through the employment of a pair of focus lenses 10A and 105, thecathode emissive surface, corresponding approximately to the objectplane, may be located at about the focal point of the first focus lens10A. Thus, for each lens a f and C', C, This may be contrasted withusing a single focus lens for focusing the cathode object at the imageplane where to do so the object plane and image plane must be spaced atan appreciably greater distance than the focal length, so that a f andC, C, From the above consideration, spherical aberration of the systemis reduced by increasing the power of focus coils 10A and 1013, withincertain limiting factors. With respect to coil 10A, the limiting factorsare principally the physical size and configuration of the anodestructure. With respect to coil 108, the limiting factors are theplacement of the deflection coils 11 and the requirement for deflectingthe beam over a wide area. Where a reflective readout is employed, as inthe embodiment of FIG. 5, a further limiting factor is presented inpositioning of the electron detector in the region above the storagemedium.

In several exemplary operable embodiments of the electron opticsstructure, each of the focus coils 10A and 108 were identical and hadthe following specifications: the bore radius R 13/16 in., and the ratioS/D 3/13, where S is the gap width and D the bore diameter. The spacingof the cathode needle 51 to the plane 61, which is the dimension k inFIG. 6, was 1.5 in., the exact dimension having been dictated primarilyby the length of the anode electrode 9. The planes 61 and 63 were spacedapart by 3.5 in., dimension 1 in FIG. 6, which is sufficient toaccommodate placement of the cores but is not considered to be acritical dimension. The coil 10A was provided with ampere turns NI z 455at SKV, and NI 640 at 10 KV C, 4.85.

With the storage medium 1 spaced 1 in. from the plane 63, dimension 0 inFIG. 6, there were provided ampere turns N] z 570 at an acceleratingvoltage of SKV, and N1 810 at IOKV C, 1.85. A spot size of 979 Adiameter was achieved. Power density at the focused spot was measured at6.64 X 10 watts per sq. cm. at 10 KV.

With the storage medium spaced 1.5 in. from the plane 63 NI 455 at SKV,and N1 640 at IOKV. C, 4.85. A spot size of 1,058 A. diameter wasachieved. Power density at the focused spot was measured at 5.69 X 10watts per sq. cm. at SKV and 1.14 X 1 ()-watts per sq. cm. at IOKV.

It. may be appreciated that the angle of convergence of the beam at thestorage medium surface is an inverse function of the spacing of themedium 1 and the plane 63. The beam spot size is a function of a and C,,and may be expressed as where d, is the ideal spot size with zero error,and a is the half angle of the converging beam. In determining thespacing between the medium 1 and the plane 63, conflicting constraintsare present. C, decreases and or increases as the spacing is reduced.The selected spacing is optimized with respect to these properties, aswell as the requirement for scanning over a relatively large area.

The dimensions of the coils presented above are primarily for purposesof example and not intended to be limiting. Other size coils may andhave been employed, with the electrical parameters appropriatelymodified to provide operation in accordance with the present teachings.

During a writing operation the modulation coils 12 drive the beam alonga single axis in the X-Y plane, so as to shift the central axis of thebeam between a position in the center of the limiting aperture 58 and aposition offset from the center where the focused beam is partiallyblocked by the limiting apertu'red electrode 57. The beam is shifted asa function of the modulation signal. With the central axis of the beamat the center of the limiting aperture 56 the focused beam spot is ofmaximum current density and will readily machine the storage material.At the offset position, the focused beam spot current density is reducedsufficiently so that no machining of the storage material can occur.Thus as the beam is scanned along the data lines by the deflection coils11, the current intensity at the focused spot is modulated and datathereby written.

During a readout operation the modulation coils 12 are employed tofixedly bias the beam in the offset position so that the beam ispartially blocked by the limiting apertured electrode 57 for fixedlyreducing the current density of the focused spot. The beam of thereduced current density is scanned along the data lines by thedeflection coils 11 for providing readout of the stored data withouteffecting any physical change or destruction of said stored data.Alternatively, the anode voltage can be reduced during readout forreducing power density at the focused spot.

In FIG. there is illustrated in a partially broken away perspective viewa further embodiment of a storage system in accordance with theinvention employing a large area storage medium 71 composed of many datablocks 72 for providing a total storage capacity several orders ofmagnitude greater than that of embodiment of FIG. 1. A single data blockcorresponds to the stor age area of the medium 1 in the embodiment ofFIG. 1. In the embodiment of FIG. 9, the total storage area of thestorage medium 71 is'a plane surface about 210 X 210 square mm.,providing about 44,000 data blocks and a total storage capacity of 10"bits. The data blocks are arranged in column and row configuration, onlyan exemplary number being shown in the partial plan view of FIG. 11.

The electron optics corresponds to the structure of FIGS. 6 and 7andcomparable components are similarly identified but with an addedprime notation. Accordingly, the electron emission system 3 and theelectron optical system 4 are identical to the previously consideredembodiment. The input network 13',-output network 17 and logic network18 may be similar to that of FIG. 1. Readout is preferably. by means ofa reflective structure such as shown in FIG. 5 employing the electrondetector 43' positioned above the storage surface. However, atransmissive readout similar to FIG. 1 may also be employed, requiring asuitable storage area electron detector structure for supporting thestorage medium. I

The storage medium 71 is mounted on a movable substrate 73 forpositioning in both the X andY directions. Movement of the substrate 73is provided by a pair of motor drive means74 and 75 located outside ofthe vacuum. Drive means 74 and 75 may include motors of conventionaltype which position the substrate 73 with an accuracy of :L- l r'nilQAvariable reactance stepping motor is suitable. Means 74through a lineardrive shaft76 drives the substrate 73 in the X direction,

The invention has been described with respect to a number of specificembodiments primarily for the purpose of clear and complete disclosure.It should be recsion density and a long lifetime. The combination of thedescribed electron optical system and the electron emission systemproduces at the target a focused beam spot of extremely small dimensionsand high current density. Conceivably, similar operation may be achievedby the present electron optical system in combination with otherelectron emission systems exhibiting similar characteristics of highemission density, stability and low long lifetime, in a medium vacuum.For

1 example, a hafnium coated tungsten cathode or a Ianand means 75through a lineardrive shaft 77drive's the substrate 73 in the Ydirection. The drive means74 and 75 may each include a motiontranslation mechanism, such as a conventional ball screw-ball nutarrangement, for converting the motors rotational motion to the linearmotion of the. drive shafts. A bellows such as shown at 78 provides avacuum seal around the drive shafts 76 and 77 while accommodating theirlinear motion.

Accordingly, in the operation of the system of FIG. 10, the motor drivemeans 76 and 75, under control of the logic network 18', provideindexing of individual data blocks 72 with respect to the electron beamstructure and the electron beam. Upon a selected data block beingindexed, the electron beam may provide write and readout operationsprecisely as described with respect to the previous embodiment of theinvention.

thanum hexaboride cathode are believed to have the inherent propertiesfor such operation, although to data are not known to have been suitablydeveloped toward this use.

Further, within the concepts of present invention, the described storagesystems may employ for storing data a material whose physical state orproperties, other thanor in addition to volume, are capable of beingchanged by a high power density beam, which change of state orpropertiescan be detected by a readout beam. In addition, the invention isconsidered toem-' body use of a storage-material capable of selectiveera'- sure by an electron beam.

It is also noted that the electron beam structure described herein mayhave useful application to other information storage systems, forexample, to micromachining and micro-etching operations in the field ofmicroelectronics.

What is claimed as new and desired to be secured by Letters Patent inthe United States is:

1. An electron-beam high densitystorage system providing storage ofinformation on a storage medium and subsequent retrieval of saidinformation, comprising:

- a. an evacuated chamber,

b. a storage medium modifiable upon electron beam impingement, 1

c. cathode means within said chamber including a 1 rigidly supportedcathode needle'structure', the tip of which provides an extremelysmalldimensioned emissive surface, r

d. field means within said chamber for providing a generally radialelectric field centered about the cathode needle emissive tip with 'asufficiently high electric field gradient in the vicinity of saidemissive tip for field emission, ssid cathode means and said field"means producing a high density emission current formed into a beamhaving a divergent configuration,

. a first focus lens for transposing the divergent configuration of saidbeams into a convergent configuration, j

f. a second focus lens for transposing the eollima ted configuration ofsaid beam into a convergent configuration, said two lenses therebyhaving-minimalspherical aborration and, the virtual image ofsaid tipbeing thereby focused on the surface of said storage medium as anextremely small spot with high current density, said storage mediumcomprising an array of discrete data blocks, each of limited surfacearea, A

g. mechanical drive means for moving said storage medium so as to singlyposition selected data blocks under the focused beam,

h. deflection means for deflecting said beam over the surface area ofsaid selected data blocks,

i. modulation means for modulating the beam as it is deflected over thelimited surface area of said selected data blocks for writinginformation upon said storage medium as a function of the modulations,and

j. readout means within said chamber for retrieving information that hasbeen written.

2. A storage system as in claim 1 wherein said cathode means includes afilamentary heating means for aiding emission, and means forcontinuously supplying a very thin coating to said tip of a materialthat reduces the work function at said tip.

3. A storage system as in claim 2 wherein said emissive tip is in aplane at about the focal point of said first lens, and said storagemedium surface is in a plane at about the focal point of said secondlens so as to reduce effective spherical aberration of said lenses.

4. A storage system as in claim 3 wherein said field means includes agrid electrode having anaperture through which said cathode needleprotrudes, and an anode electrode positioned forward of said cathodeemissive tip, having an aperture coaxially related to said cathodeneedle through which the central portion of said beam is directed. r

5. A storage system as in claim 4 wherein said modulation means shiftsthe beam so as to be recurrently intercepted by said apertured electrodestructure as a function of the modulations, whereby the emission densitymay be maintained constant as the current density at the storage mediumsurface is varied and wherein said readout means includes an electrondetector which responds to electrons received from said storage mediumin response to impingement by said beam when operated at reduced currentdensities.

6. A storage system as in claim 5 in which the focused beam spotpossesses a diameter on the order of 0.1 microns and less with a currentdensity on the order'of 1,000 amperes per sq. cm. and greater.

7. A storage system as in claim 6 in which said storage medium iscomposed of a'selenium composition, portions of which are selectivelyvaporized by the focused beam during the writing operation.

8. An electron beam high density storage system providing storage ofinformation on a storage medium and subsequent retrieval of saidinformation, comprising:

a. an evacuated chamber,

b. a storage medium modifiable upon electron beam impingement c. cathodemeans within said chamber including a rigidly supported cathode needlestructure, the tip of which provides an extremely small dimensionedemissive surface,

d. field means within said chamber for providing a generally radialelectric field centered about the cathode needle emissive tip with a'sufficiently high electric field gradient in the vicinity of saidemissive tip for field emission, said cathode means and said field meansproducing a high density emission current formed into a beam having adivergent configuration, a first focus lens for transposing thedivergent configuration of said beam into a collimated configuration, asecond focus lens for transposing the collimated configuration said twolenses thereby having minimal spherical aberration and, of said beaminto a convergent configuration, the virtual image of said tip beingthereby focused on the surface of said storage medium as an extremelysmall spot with high current density, g. deflection means for deflectingsaid beam over said storage medium surface, a

h. modulation means for modulating the beam as it is deflected over thestorage medium surface for writing information upon said storage, mediumas a function of the modulations, and

i. readout means within said chamber for retrieving information that hasbeen written.

9. A storage system as in claim 8 wherein said cathode means' includes afilamentaryiheating means for aiding emission, and means-forcontinuouslysupplying a very'thincoating to said tip of a material thatreduces the work function at said tip.

10. A storage system as in claim 9 wherein said emissive tip is'in aplane at about the focal point of said first lens, and said storagemedium surface is in a plane at about the focal point of said secondlens so as to reduce efi'ective spherical aberration of said lenses.

11. A storage system as in claim 10 in which said field means includes agrid electrode having an aperture through which said cathode needleprotrudes and an anode electrode positioned forward of said emissive tiphaving an aperture coaxially related to said cathode needle,'throughwhich the central portion of said beam is directed.

12'. A storage system as in claim 11 wherein said first and second focuslenses each comprise a magnetic coil wound about the circumference ofsaid chamber.

13. A storage system as in claim 12 whereinsaid deflection means ispositioned between said second focus lens and said storage medium, andwherein said modulating means shifts the beam so as to be recurrentlyintercepted by said apertured electrode structure as a function of themodulations, whereby the emission density may be maintained constant asthe current density at the storage medium surface is varied.

14. A storage system as in claim 13 wherein said readout means includesan electron detector which responds to electrons received from saidstorage medium in response to impingement by said beam when operated atreduced current densities.

15. A storage system as in claim 14 in which the focused beam spotpossesses a diameter on the order of 0.1 microns and less with a currentdensity on the order of 1,000 amperes per sq. cm. and greater.

16. A storage system as in claim 15 in which said storage medium iscomposed primarily of a selenium composition, portions of which areselectively vaporized by

1. An electron beam high density storage system providing storage ofinformation on a storage medium and subsequent retrieval of saidinformation, comprising: a. an evacuated chamber, b. a storage mediummodifiable upon electron beam impingement, c. cathode means within saidchamber including a rigidly supported cathode needle structure, the tipof which provides an extremely small dimensioned emissive surface, d.field means within said chamber for providing a generally radialelectric field centered about the cathode needle emissive tip with asufficiently high electric field gradient in the vicinity of saidemissive tip for field emission, ssid cathode means and said field meansproducing a high density emission current formed into a beam having adivergent configuration, e. a first focus lens for transposing thedivergent configuration of said beams into a convergent configuration,f. a second focus lens for transposing the collimated configuration ofsaid beam into a convergent configuration, said two lenses therebyhaving minimal spherical aborration and, the virtual image of said tipbeing thereby focused on the surface of said storage medium as anextremely small spot with high current density, said storage mediumcomprising an array of discrete data blocks, each of limited surfacearea, g. mechanical drive means for moving said storage medium so as tosingly position selected data blocks under the focused beam, h.deflection means for deflecting said beam over the surface area of saidselected data blocks, i. modulation means for modulating the beam as itis deflected over the limited surface area of said selected data blocksfor writing information upon said storage medium as a function of themodulations, and j. readout means within said chamber for retrievinginformation that has been written.
 2. A storage system as in claim 1wherein said cathode means includes a filamentary heating means foraiding emission, and meAns for continuously supplying a very thincoating to said tip of a material that reduces the work function at saidtip.
 3. A storage system as in claim 2 wherein said emissive tip is in aplane at about the focal point of said first lens, and said storagemedium surface is in a plane at about the focal point of said secondlens so as to reduce effective spherical aberration of said lenses.
 4. Astorage system as in claim 3 wherein said field means includes a gridelectrode having an aperture through which said cathode needleprotrudes, and an anode electrode positioned forward of said cathodeemissive tip, having an aperture coaxially related to said cathodeneedle through which the central portion of said beam is directed.
 5. Astorage system as in claim 4 wherein said modulation means shifts thebeam so as to be recurrently intercepted by said apertured electrodestructure as a function of the modulations, whereby the emission densitymay be maintained constant as the current density at the storage mediumsurface is varied and wherein said readout means includes an electrondetector which responds to electrons received from said storage mediumin response to impingement by said beam when operated at reduced currentdensities.
 6. A storage system as in claim 5 in which the focused beamspot possesses a diameter on the order of 0.1 microns and less with acurrent density on the order of 1,000 amperes per sq. cm. and greater.7. A storage system as in claim 6 in which said storage medium iscomposed of a selenium composition, portions of which are selectivelyvaporized by the focused beam during the writing operation.
 8. Anelectron beam high density storage system providing storage ofinformation on a storage medium and subsequent retrieval of saidinformation, comprising: a. an evacuated chamber, b. a storage mediummodifiable upon electron beam impingement c. cathode means within saidchamber including a rigidly supported cathode needle structure, the tipof which provides an extremely small dimensioned emissive surface, d.field means within said chamber for providing a generally radialelectric field centered about the cathode needle emissive tip with asufficiently high electric field gradient in the vicinity of saidemissive tip for field emission, said cathode means and said field meansproducing a high density emission current formed into a beam having adivergent configuration, e. a first focus lens for transposing thedivergent configuration of said beam into a collimated configuration, f.a second focus lens for transposing the collimated configuration saidtwo lenses thereby having minimal spherical aborration and, of said beaminto a convergent configuration, the virtual image of said tip beingthereby focused on the surface of said storage medium as an extremelysmall spot with high current density, g. deflection means for deflectingsaid beam over said storage medium surface, h. modulation means formodulating the beam as it is deflected over the storage medium surfacefor writing information upon said storage, medium as a function of themodulations, and i. readout means within said chamber for retrievinginformation that has been written.
 9. A storage system as in claim 8wherein said cathode means includes a filamentary heating means foraiding emission, and means for continuously supplying a very thincoating to said tip of a material that reduces the work function at saidtip.
 10. A storage system as in claim 9 wherein said emissive tip is ina plane at about the focal point of said first lens, and said storagemedium surface is in a plane at about the focal point of said secondlens so as to reduce effective spherical aberration of said lenses. 11.A storage system as in claim 10 in which said field means includes agrid electrode having an aperture through which said cathode needleprotrudes and an anode electrode positioned forward of said emissive tiphaviNg an aperture coaxially related to said cathode needle, throughwhich the central portion of said beam is directed.
 12. A storage systemas in claim 11 wherein said first and second focus lenses each comprisea magnetic coil wound about the circumference of said chamber.
 13. Astorage system as in claim 12 wherein said deflection means ispositioned between said second focus lens and said storage medium, andwherein said modulating means shifts the beam so as to be recurrentlyintercepted by said apertured electrode structure as a function of themodulations, whereby the emission density may be maintained constant asthe current density at the storage medium surface is varied.
 14. Astorage system as in claim 13 wherein said readout means includes anelectron detector which responds to electrons received from said storagemedium in response to impingement by said beam when operated at reducedcurrent densities.
 15. A storage system as in claim 14 in which thefocused beam spot possesses a diameter on the order of 0.1 microns andless with a current density on the order of 1,000 amperes per sq. cm.and greater.
 16. A storage system as in claim 15 in which said storagemedium is composed primarily of a selenium composition, portions ofwhich are selectively vaporized by the focused beam during the writingoperation.